Shaped part of an ultra high molecular weight polyethylene

ABSTRACT

Shaped parts of ultra high molecular weight polyethylene (UHMWPE) are made by a process comprising melt processing, wherein a) the UHMWPE has a weight average molecular weight (Mw) of at least 1*106 g/mol, b) during the shaping the storage plateau modulus of the UHMWPE (G*) is kept at a value of at most 1.5 MPa, c) whereafter, before the cooling, the G* is raised to its final value. The shaped parts may advantageously be used in medical applications, e.g., as an element of a hip or knee prosthesis.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 10/561,290filed on Jan. 25, 2006, which in turn is the US national phase ofinternational application PCT/NL2003/000473 filed Jun. 26, 2003 whichdesignated the U.S., the entire content of each being herebyincorporated by reference.

FIELD OF INVENTION

The present invention relates to a process for the preparation of ashaped part of an ultrahigh molecular weight polyethylene (UHMWPE) byheating the UHMWPE to a temperature above the melting temperature,shaping the resulting melt, and cooling the melt to a temperature belowthe melting temperature. The invention further relates to a shaped partobtainable with this process, and to the use thereof, especially inmedical applications.

BACKGROUND AND SUMMARY OF INVENTION

Such a process is known from WO 03/037590. In this publication a shapedpart of UHMWPE is prepared wherein the UHMWPE is annealed for at leastone hour at a temperature of between 130 and 160° C. in order to obtaina low chain entanglement and thus processability.

The processability of a synthetic polymer is often a compromise betweenthe ease of processing and desired product properties. Processing routesconventionally applied in the polymer industry are injection moulding,extrusion and blow moulding. All these routes start from a melt of thepolymer. Melt properties are mostly affected by the molecular mass ofthe polymer.

For a melt consisting of relatively low molecular mass polymer(M_(W)<M_(C)) there is a direct proportionality between zero-shearviscosity (η₀) and molecular mass, whereas for a melt consisting of highmolecular mass polymer (M_(W)>M_(C)) the viscosity depends much morestrongly on the molecular mass (η₀˜(M_(W))^(3,4)). Herein is M_(W) theweight averaged molecular mass and M_(C) the critical molecular mass,which is related to the shortest polymer chain length able to form anentanglement. This difference in viscosity of the two molecular massregimes is due to the ability of long chains to entangle, which imposesa restriction on the flowability of a melt.

The motion of chains within a highly entangled melt is described by thereptation model introduced by De Gennes in J. Chem. Phys. 55, p. 572(1971). In this model a chain within a melt moves in worm-like fashionthrough a virtual tube, which is delineated by entanglements formed byneighbouring chains. The time needed for a chain to renew its tube(reptation time), i.e. to change its position within the melt is alsohighly dependent on molecular mass (τ₀˜M_(W) ³). These fundamentalrestrictions make high molecular mass polymers rather intractable viaconventional processing routes. On the other hand, final properties liketenacity, strength and wear improve with increasing molecular mass.Superior properties are necessary to meet the requirements of demandingapplications.

The discrepancy between intrinsic properties related to high values ofmolecular mass and insufficient product performance due to difficultiesin processing is encountered in UHMWPE as well as in other polymers ofvery high molecular mass. UHMWPE is a linear grade polyethylene, as ishigh-density polyethylene (HDPE), but possesses a weight averagemolecular mass (Mw) of at least 7.5*10⁵ g/mol (according to ASTM D4020).Preferably the UHMWPE has a weight average molecular mass of at least3*10⁶ g/mol, because of excellent mechanical properties.

The density of the entanglements seems to play a prominent role in theprocess of forming a shaped part from the melt. The effect ofentanglement density was confirmed by drawing experiments onsingle-crystal mats from UHMWPE, as reported by T. Ogita et al. inMacromolecules 26, p. 4646 (1993). In the case of melt crystallisedUHMWPE, entanglements are trapped upon crystallisation and limit theextent to which the chains can be drawn. On the other hand,crystallisation of long molecular chains out of semi-dilute solutionsleads to a much less entangled system and this enables these materialsto be drawn below the melting temperature. It has always been believedthat once a disentangled state of UHMWPE has been achieved, theformation of entanglements within the melt will be very slow, due to along reptation time, and consequently one would be able to benefit froma disentangled state during processing. Experimental results howevershowed that highly disentangled solution crystallised films of UHMWPE,which are drawable below the melting temperature lose their drawabilityimmediately upon melting. This phenomenon has been associated with thatof “chain explosion”, as experimentally assessed by P. Barham and D.Sadler in Polymer 32, p. 939 (1991). With the help of in-situ neutronscattering experiments they observed that the chains of highlydisentangled folded chain crystals of polyethylene increase the radiusof gyration instantaneously upon melting. Consequently the chainsentangle immediately upon melting, which causes the sudden loss inprocessability and drawability once the sample has been molten.

These results showed that the fundamental restrictions resulting fromthe strong dependence of the zero-shear viscosity on molecular masscannot be easily overcome. Simple disentanglement of the chains prior tomelting will not lead to a less entangled melt and accordingly it cannotbe used to improve the melt processability of UHMWPE.

The objective of the present invention is to provide a process for themanufacture of a shaped part of ultra high molecular weight polyethylene(UHMWPE) comprising melt processing, which part shows goodprocessability below its melting point, and wherein the state ofdisentanglement is maintained long enough in the melt to process it asan only partly entangled melt.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a plot of G*-measurements versus time at a heating rate e of20K/minute obtained from the data of Examples V to VIII and ComparativeExperiment A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention this objective is achieved with aprocess wherein:

a) the UHMWPE has a weight average molecular weight (Mw) of at least1*10⁶ g/mol,b) during the shaping the storage plateau modulus (G*) of the UHMWPE iskept at a value of at most 1.5 MPa,c) whereafter, before the cooling, the G* is raised to its final value.

The G* is determined under nitrogen with a rotational viscometer(rheometer), using a parallel plate geometry (diameter 12 mm). Samplesfor rotational viscometry were made using compression moulding. Thecircular preforms having a diameter of about 8 mm and a thickness of 1mm were moulded at a temperature of 50° C. Each two minutes the pressingforce was increased in seven steps to a maximum of 50 kN. Oscillatoryshear measurements were performed in a frequency range of 0.1-100 Hz at180° C.

With the process according to the invention a shaped part can be made bymelt processing out of ultra high molecular weight polyethylene. Thepart thus formed is still highly drawable below its melting point, whichindicates that, even though the UHMWPE under the specified conditions isprocessed in the melt, it still has a low entanglement density.

Surprisingly it has been found that when using the specified UHMWPE,under the specified conditions, a separate annealing step can beomitted. The process of increasing entanglement is under the conditionsof the present invention so much retarded, that a disentangledthermodynamically metastable melt is present for a sufficient large timeto process the material.

The time during which the entanglement increases, resulting in abuild-up of the storage plateau modulus G* to a final value of around2.0 MPa (which is indicative for a highly entangled UHMWPE), is in mostcases depending on the heating rate (Θ) of the polymer. When adependence is observed, the build-up time increases with a decreasing Θ.If an extended processing time window is required, it is preferred thatΘ is at most 5 K/minute; even more preferred at most 1 K/minute.

Also the starting value of G* is therefore of importance. The lower theG* of the used UHMWPE, the longer it takes to achieve the G*=2.0 MPavalue. Therefore it is preferred that the initial value of the G* of theused UHMWPE is at most 0.75 MPa. For a given polymerization with aspecific catalyst system, the resulting G* starting value is typicallylower with lower polymerization temperature. Although from a mechanicalproperties point of view, the desired G* end-value is 2.0 (fullyentangled material), for processing the UHMWPE the G* must be below 1.5MPa, more preferably below 1.2 MPa.

The G* build-up time can be extended or reduced by polymerizationtemperature and/or processing heating rate. In fact, the slower thebuild up of the G* value during the shaping, the better it is for theprocessability of the UHMWPE, as it retards the increase inentanglement. Therefore it is preferred that the speed (v) at which theG* builds up during shaping, is less than 3 MPa/hour, more preferredeven less than 0.5 MPa/hour.

After the shaping, the value of the then achieved G* is raised to itsfinal value of around 2.0 MPa. The speed at which this is done, can beas high as the situation allows. This can be achieved by increasing theheating rate. The final temperature should preferably not exceed 450 K.

An additional annealing step, as taught in WO 03/037590, will increasethat build-up time and therefore enlarge the processing window betweenthe initial G* and the highly entangled stage (having a G* of around 2.0MPa). Preferably this annealing is performed at a temperature of notless than 398 K and not more than 410 K. For more details on thisannealing step, the reader is referred to the above indicated patentpublication.

The UHMWPE that is used in the process of the present invention has tofulfill at least the following criteria:

the weight average molecular weight (Mw) is at least 1*10⁶,the storage plateau modulus (G*) is kept at a value of at most 1.5 MPaduring the shaping.

This requires specific conditions for the polymerization process inwhich the UHMWPE is made:

the temperature at which the polymerization takes place is between 225and 325 K, more preferably between 260 and 305 K. The higher thetemperature, the higher the G*-value of the resulting UHMWPE;

The catalyst is an unsupported single-site catalyst or a mixture ofunsupported single-site catalysts, known for the polymerization ofethylene to UHMWPE;

The catalyst concentration is low: less than 1*10⁻⁴ mol/l, morepreferred less than 10⁻⁶ mol/l.

The polymerization temperature is lower than 325 K and preferably lowerthan 300 K.

As of this nature, the polymerization takes place initially in solutionand after formation and precipitation of polymer in suspension.

The UHMWPE can either be a homopolymer of ethylene, or a copolymer ofethylene with a comonomer in the form of another a-olefin or a cyclicolefin. Examples of such a comonomer are: propylene, 1-butene,1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, (substituted)norbornenes and the amount of incorporated comonomer should besufficiently low to ensure that the polymer still crystallizes underpolymerization conditions. Through simple experimentation the skilledman is able to find the suitable process conditions that result in thepreparation of the UHMWPE fulfilling the above stipulated criteria.Preferably, the MWD of the UHMWPE lies between 1.2 and 3.0, morepreferred between 1.2 and 2.5.

The invention also relates to a shaped part that is obtainable with theprocess of the present invention, as well as to the use of the shapedpart. The shaped part can be in the form of a filament, a film, amoulded or extruded article. The processes to obtain such a shaped partfrom the polymer meet are known to the skilled artisan.

As the shaped parts according to the present invention have enhancedtoughness, wear and abrasion resistance, reduced oxygen permeability,and are essentially grain boundary free, they are very well suited forthe use in a medical application. Preferably the shaped part can thus beused as an element of a hip or knee prosthesis.

Also other uses, wherein the improved physical and mechanical propertiesof the shaped are applicable, can be referred to, like the use of theUHMWPE-based shaped part in bearings.

The invention is elucidated with the following non-limiting Examples andcomparative experiments.

Examples I-IV and Comparative Experiment A

An UHMWPE was prepared according to Example XXV of EP-A-1,057,837. Theproduct had an initial G* of 0.6 MPa, an MWD of 2 and an Mw of 4*10⁸(according to ASTM D4020).

A sample thereof was molten in a rheometer (Ares 3LS-4A, RheometricsInc.) and heated with different heating rates from 398 to 418 K. Theincrease of the storage plateau modulus G* (recorded at 10 rad/s) wasfollowed in relation to different heating rates Θ.

The results are given in Table 1.

TABLE 1 Example/ Θ G* after 8,000 sec. G* after 16,000 sec. Exp (K/min)(MPa) (MPa) I 0.25 0.7 0.8 II 1.00 1.0 1.1 III 5.00 1.3 1.5 A 20.00 1.71.8

Example IV and Comparative Experiment B

The UHMWPE of Example I was heated to a temperature of 453 K atdifferent heating rates Θ. The G*-value was recorded (at 10 rad/s) vs.time. The results are given in Table 2.

TABLE 2 Example/ Θ G* after 10,000 sec. G* after 80,000 sec. Exp (K/min)(MPa) (MPa) IV 0.25 0.8 0.9 B 20.00 1.7 1.9

The products from comparative experiments A and B had a poordrawability, resulting from their high G*-value. The products from theExamples 1-IV were very well drawable, due to their low G*-value (5.1.5MPa).

Examples V TO VIII Experimental

All air- and/or water-sensitive activities were performed under an argonatmosphere using Schlenk techniques or in a conventional nitrogen-filledglove box (Braun MB-150 GI). Methylalumoxane was purchased from WITCOGmbH as a 10 wt % toluene solution. Ethylene was obtained from AirLiquide. Petroleum-ether (40-60), used as the polymerization solvent,was dried over Al₂O₃. The catalysts [3-tBu-2-O—C₆H₃CH═N(C₆F₅)]₂TiCl₂ and(C₅Me₅)₂Sm(THF)₂ were synthesized according to the literature. Themolecular weight and molecular weight distribution was measured at 135°C. by gel-permeation chromatography (GPC; GPC210, Polymer Labs) using1,2,4-trichlorobenzene as solvent.

Polymerizations:

Examples V and VI Catalyst is (C₅Me₅)₂Sm(THF)₂

The polymerizations were carried out at −10 and 0° C., respectively,under atmospheric pressure using a 2000 ml round bottom flask, equippedwith a thermocouple and a mechanical stirrer. Petroleum ether (1000 ml)was introduced to the argon-purged reactor after which the solvent wassaturated by bubbling ethylene into the solution for 45 minutes at −10°C. and 0° C. resp. The polymerization was initiated by addition of atoluene solution of catalyst (2.8 μmol) into the reactor while stirringvigorously. The same amount of catalyst was added ten times at aninterval of one minute. The polymerization was quenched after 15 minwith methanol. The solid UHMWPE was recovered by filtration, washed withwater and acetone and dried (vacuum oven 60° C., overnight).

Examples VII and VIII Catalyst is [3-tBu-2-O—C₆H₃CH═N(C₆F₅)]₂TiCl₂

The polymerizations were carried out under atmospheric pressure using a2000 ml round bottom flask, equipped with a thermocouple and amechanical stirrer. Petroleum ether (1000 nil) was introduced to theargon-purged reactor after which the solvent was saturated by bubblingethylene into the solution for 30 minutes at −10° C. and +20° C.,respectively. The polymerization was initiated by addition of a toluenesolution of methylalumoxane (20 ml) after which a toluene solution ofthe catalyst (1 μmol) was introduced into the reactor while stirringvigorously. After 20 minutes the ethylene feed was stopped and isobutylalcohol was added to terminate the polymerization. HCl and water wereadded to the resulting mixture. The solid UHMWPE was recovered byfiltration, washed with water and acetone and dried (vacuum oven 60° C.,overnight).

Synthesis Example Catalyst temp (° C.) Mw Mn MWD V Sm −10 1,225,500607,700 2.0 VI 0 2,040,500 832,856 2.5 VII Ti −10 1,066,000 840,600 1.3VIII +20 1,089,000 794,800 1.4

The results of the G*-measurements are given in FIG. 1 at page 12(determined at a heating rate .THETA. of 20 K/minute).

1. An essentially grain boundary free shaped part of an ultrahighmolecular weight polyethylene (UHMWPE) made by a process comprisingheating the UHMWPE to a temperature above the melting temperature,shaping the resulting melt, and cooling the melt to a temperature belowthe melting temperature, wherein a) the UHMWPE has a weight averagemolecular weight (Mw) of at least 1*10⁶ g/mol, b) during the shaping thestorage plateau modulus (G*) of the UHMWPE is kept at a value of at most1.5 MPa, c) whereafter, before the cooling, the G* is raised to itsfinal value.
 2. A shaped part according to claim 1, wherein the UHMWPEis heated at a heating rate (Θ) which is at most 1 K/minute, as of atemperature of 350K.
 3. A shaped part according to claim 1, wherein theUHMWPE is heated at a heating rate (Θ) which is at most 5 K/minute.
 4. Ashaped part according to claim 1, wherein the UHMWPE has a MWD ofbetween 1.2-3.0, inclusive.
 5. A shaped part according to claim 1,wherein the initial value of G* is at most 0.75 MPa.
 6. A shaped partaccording to claim 1, wherein G* builds up to a value of 1.5 MPa at aspeed (Ψ) less than 3 MPa/hour.
 7. A shaped part according to claim 6,wherein Ψ is less than 0.5 MPa/hour.
 8. A shaped part according to claim1, wherein the UHMWPE is obtained through a solution or suspensionpolymerization at a temperature of between 225 and 325 K, using anunsupported catalyst in a concentration of less than 1×10⁻⁴ mol/L.
 9. Ashaped part according to claim 1, wherein the UHMWPE is either ahomopolymer of ethylene, or a copolymer of ethylene with anotherα-olefin or cyclic olefin.
 10. A shaped part according to claim 8,wherein the polymerisation takes place at a temperature between andinclusive 260 and 305 K.
 11. A shaped part according to claim 1, whereinthe UHMWPE is annealed during the heating, at a temperature of not lessthan 398 K and not more than 410 K.
 12. A medical device which comprisesa shaped part according to claim
 1. 13. A medical device according toclaim 12, wherein the shaped part is an element of a hip or kneeprosthesis.
 14. A shaped part comprised of an ultrahigh molecular weightpolyethylene (UHMWPE) made by a process comprising the steps of: (a)heating at a heating rate (O) which is at most 1 K/minute an ultrahighmolecular weight polyethylene (UHMWPE) having a weight average molecularweight (Mw) of at least 1*10⁶ g/mol and having an initial storageplateau modulus (G*) value of at most 0.75 MPa to a temperature abovethe melting temperature of the UHMWPE to form a processable meltthereof; (b) allowing the initial G* value of the UHMWPE melt to buildat a speed (Ψ) of less than 3 MPa/hour to a processing G* value of atmost 1.5 MPa; (c) shaping the melt of the UHMWPE while maintaining theprocessing G* value of at most 1.5 MPa to form a shaped part thereof;and thereafter (d) prior to cooling, raising the G* value of the shapedpart formed according to step (c) from a G* value of at most 1.5 MPa toa final G* value of about 2.0 MPa; and then (e) cooling the shaped part.15. A shaped part according to claim 14, wherein step (a) is practicedby heating the UHMWPE at a heating rate (Θ) which is at most 5 K/minute.16. A shaped part according to claim 14, wherein the UHMWPE has amolecular weight distribution (MWD) of between 1.2-3.0, inclusive.
 17. Ashaped part according to claim 14, wherein step (b) is practiced byallowing the initial G* value of the UHMWPE to build at a at a speed (T)of less than 0.5 MPa/hour.
 18. A shaped part according to claim 14,wherein the UHMWPE is the solution or suspension polymerization productobtained at a polymerization temperature of between 225 and 325 K, usingan unsupported catalyst in a concentration of less than 1×10⁻⁴ mol/L.19. A shaped part according to claim 18, wherein the polymerisationtakes place at a temperature between and inclusive 260 and 305 K.
 20. Ashaped part according to claim 14, wherein the UHMWPE is either ahomopolymer of ethylene, or a copolymer of ethylene with anotherα-olefin or cyclic olefin.